1. Field of the Invention
The field of this invention relates to electromagnetic acoustic transducers usable with substantially cylindrical objects, and methods for determining resonant frequencies and identifying physical properties, particularly flaws, of the substantially cylindrical objects using electromagnetic acoustic transducers with coils recessed into parallel arrays of magnets arranged in a radial fashion transverse to the longitudinal axis of the cylindrical objects.
2. Description of the Prior Art
Inspections according to and as required by government regulations and industry standards are commonly performed on commercially manufactured products including steel and aluminum pipes and tubing and also including elongated, substantially cylindrical bar stock, such as those used in pressurized applications, and also including elongated, substantially cylindrical bar stock, such as those used in high torque applications, e.g., fastenings. The subjects of these regulations and standards are the qualitative and quantitative standards controlling the properties of materials, such as strength, granularity and the presence and severity of flaws.
Clearly, the presence of certain defects can adversely affect the safety and structural integrity of the finished product. It is therefore preferred that the flawed portions of these metallic stocks and commercially manufactured products be economically identified and removed with the flawed portions of these materials recycled. In addition, the need to correct and improve processes and machinery by identifying flaw-producing causal relationships motivates actions that subject the removed portions to further inspections prior to recycling.
It is well known that in the fabrication and refinement of metallic articles, the foundry process itself, as well as errant machinery and other equipment used in the fabrication and refinement can produce one or more unacceptable flaws in the product articles. Present common practice in accordance with current quality control programs places an entire production run subject to rejection due to flaws in a small, but statistically significant, number of parts. There exists economic motivation to detect such flaws directly and thereby indirectly identify one or more problems with the manufacturing equipment or process. Once identified and corrected, manufacturers may then work to minimize the future generation of flawed metallic structures as well as the magnitude of the rejected stock under test. Varying degrees of economic benefit may be gained by the use of various test techniques in ascertaining the location of a flaw and determining the type of flaw.
Presently, there exist a variety of test techniques available and known to practitioners in the field that manufacturers may implement separately or in combination to determine flaws in metallic structures. Generally, one may dichotomize these as destructive and non-destructive test techniques. In destructive testing, a selected portion of the subject metallic item is destroyed; a portion that could very well be flawless while the remaining unselected portions of the metallic part may contain numerous unacceptable flaws. The extensibility of the results of destructive testing of a portion of the metallic item to the remainder of the item can vary greatly.
The principal advantage of non-destructive testing is that the metallic structure is examined throughout with only the flawed portion being isolated and removed, thereby leaving the unflawed portion for immediate usage. Typically, once a flawed portion in the metallic structure is determined, the testing procedure and/or device employs some means to ascertain the location of that flaw and the metallic structure is marked accordingly. With respect to non-destructive test techniques, there is a wide range of technology available including the use of eddy currents, magnetic flux leakage, x-ray, ultrasound, neutron detraction, and so forth. Where commercially practical non-destructive testing is implemented, one skilled in the art typically places one or more of devices embodying at least one of these non-destructive technologies within the production line so as to ascertain a flawed portion of a particular metallic structure during the normal production of a metallic structure.
Practitioners in the field are generally aware of the use of acoustic resonance techniques in the non-destructive testing of metallic structures. The use of acoustic resonance can offer significant advantages over other prior art types of non-destructive testing technologies. However, acoustic resonance techniques of the prior art have met with limited success due to the difficulty and expense in applying such especially with the use of contact type transducers.
Contact-type transducers, by their mechanical nature, affect the intrinsic resonance of the metallic structure since such transducers, by necessity, come into mechanical contact with the metallic structure. The affect the testing device has on the metallic part under test leads to complex signal and processing schemes to account for the variation in resonance such as taught by U.S. Pat. No. 5,062,296 to Magliori and U.S. Pat. No. 4,9766,148 to Magliori, et al. These inventions employ a ceramic piezoelectric transducer, one that is typically a contact-type of transducer. A non-destructive testing device that does not require contact with the metallic part under test would be preferred so that the metallic structures under test resonate in isolation and thereby forgo the complexities and uncertainties associable with the contact-type signal processing.
Non-destructive testing can be performed by electromagnetic acoustic transducer (EMAT) devices disclosed in U.S. Pat. No. 6,190,108 to Ohtani, et al., U.S. Pat. No. 6,164,137 to Hancock, et al., U.S. Pat. Nos. 5, 895,856, 6,119,522 and 6,170,336 all to Johnson, et al. (collectively referred to as Johnson) and in U.S. Pat. No. 5,808,202 to Passarelli. Ohtani discloses burst wave technique with an apparatus using a sheet coil. Hancock discloses a technique for tubes using dual circumferential acoustic waves produced by a transmitting EMAT and sensed by a pair of receiving EMATs. Johnson and Passarelli disclose EMATs typically in arranged one or more planes of magnets of alternating polarities arrayed in a radial fashion about a substantially annular orifice or pass-through hole and transverse to the longitudinal axis of the substantially cylindrical object under test. Electrical wire is then positioned about each magnet array. Passarelli and Johnson disclose EMATs that can provide flaw detection at various levels within a metallic structure under test. This detection in depth offers significant advantages over Hancock when moving from tubular to substantially solid objects under test.
Passarelli discloses each plane pair, together with transmitting and receiving coils and signal processing, act as the transmitting and receive circuit and provide the fields producing the resonance frequencies in the metallic structures under test. Observations gathered from field trials in both steel and aluminum mills indicate that there is an unacceptable decrease in the distance from the metallic part under test to the transducer coils. That is, the gap distance is decreased for the larger sample sizes of metallic structures of one inch or greater in diameter or maximal transverse dimension, limiting the degree to which one may locate the transmitting and receiving coils in closest practical proximity to one another, as is done for smaller diameter metallic structures such as around one-eighth to three-eights inch in diameter. On the other hand, if the gap distance is practically minimized, the electrical benefit is that the field coupling into the metallic structure is maximized and the cross-talk leaking into an adjacent transducer is minimized.
Under typical milling conditions, a metallic structure such as a pipe or tube moves at around three hundred feet per minute relative to the transducer. In the course of ordinary milling condition, the metallic structure moves laterally, or wobbles, and it is this wobble, in practice, that causes the metallic structure to mechanically contact the transducer. That is, as the gap distance decreases, the metallic structure under test is at great risk of coming into contact with the transducer causing damage to the transducer and posing a hazard to mill operations. Thus, a disadvantage of Passarelli and Johnson is that one cannot both accommodate the larger stock and ensure, under continuous mill conditions, the safe passage of the material structure in conjunction with the transducer, because the gap distance from the metallic part to the transducer coil cannot be practicably decreased within the prior art to acceptable distances.
There is another disadvantage to the aforementioned prior art arrangement in that the geometry of the coil used in conjunction with the transducer has an aspect ratio, thin and wide, that naturally causes the solenoid coil to have substantially field emanating from the coil ends along its Z axis. Increasing coil thickness to improve coil performance by tightening up the field produces marginal improvement. But the major disadvantage is that the magnets must be moved further away in order to accommodate the thicker coil. This is not a satisfactory tradeoff since the magnetic field provided by the magnets decreases as the square of the distance from the metallic structure.
An additional difficulty posed by the prior art EMAT structure is that in order to achieve a minimal level of direct coupled signal from the transmitter coil to the receiver coil, the coil turns count must be reduced to enable the coils to be spaced further apart, thus decreasing the defect size resolution. The path length over which the resonance sound field traverses is longer and therefore increase the material""s volume under resonance.
A non-destructive testing apparatus is disclosed comprised of a pair of EMATs; a first EMAT for the transmission and inducement of acoustic waves intended to establish resonances within the metallic structure under test and a second EMAT for the reception of the induced acoustical resonances. Each EMAT is comprised of an electrical coil mounted within a channel or chamber formed by the notched ends of magnets where said magnets are arranged to form an annular array in a radial fashion transverse to the longitudinal axis of the metallic structure under test. The novel recessed mounting of the coils provided by the channels or chambers substantially reduces the observed electrical cross-talk between coils over the prior art and reduces the likelihood of the coils making contact with the subject metallic structure under test while the metallic structure is in longitudinal motion relative to the coils.
In practice, a particular reference resonant frequency is known or can be calculated for an unflawed metallic structure. The transmitting coil is supplied power which when combined with the force of the magnets will cause the metallic structure to vibrate within a range that is to include a resonant frequency. As the magnetic structure passes relative to the transducers and when a flaw is detected, the resonant frequency is shifted out of the range established for an unflawed metallic structure. This shifted resonant frequency can be above the range or below the range for the unflawed metallic structure. The induced resonant frequency of the metallic structure is sensed by the receiving transducer and then by a voltage/current sensor, such as an AM detector, transmitted via an analog-to-digital converter to a computer. Associated with the transmitting transducer and receiving transducer pair is some form of optional marking device that is capable of marking the metallic structure at the point of the flaw once the resonant frequency of the flaws portion exceeds the unflawed range as determined by the computer.
The subject matter of this invention is constructed so that each array of magnets located in conjunction with each transducer has a channel, or chamber formed therein. Within this channel is located an electrical coil. This placement of the electrical coil is different from prior art where the coil was mounted on the exterior surfaces of the magnets closest to the metallic structure under test. This novel accommodation and placement of the coils within the channel or chamber formed by the magnets of the transmitting transducer and the receiving transducer: (1) permits the transmitting transducer and receiving transducer to be enclosed within a single housing and located in close proximity to the metallic structure being tested and (2) substantially reduces the cross-talk between the transmitting transducer and the receiving transducer.
The advantages of using the construction of the transducers of this invention are as follows: (1) with respect to aspect ratio, the coil aspect ratio can now be altered making the coil thicker and narrower without moving the permanent magnet array farther from the metallic structure (i.e., the same coil winding density, or turns ratio, is maintained and therefore, the coil""s inductive properties can better match those of a wider coil design); (2) with respect to tighter acoustic field dispersion, the narrower coil now produces a narrow acoustic field that is generated by the transmitter transducer and the channel within which the coil is mounted can be moved so as to maximize the detection resolution depending upon measurement requirement; (3) with respect to solenoid coil side lobe reduction, the solenoid coil that is mounted within the channel of the magnets significantly reduces cross talk, (i.e., the electromagnetic signal that directly couples from the transmitting transducer into the receiving transducer) and this mounting of the coil within a channel of the magnets effectively provides a shield to reduce this cross-talk by a factor greater than ten from prior art structures such as disclosed in U.S. Pat. No. 5,808,202 to Passarelli; (4) with respect to magnet lift-off distance, the channeled magnet structure of this invention decreases magnet lift-off distance by bringing the edges, or skirts of the magnets, closer to the metallic structure being tested by a factor equal to the coil thickness and thereby strengthens the direct current bias field supplied by the magnets by a factor of square of the distance although the magnets and coil are not inline with each other at these skirts and the additional field provided by the magnets improves transduction efficiency; (5) with respect to coil protection, the soft coil is now recessed into the relatively hard magnet skirt providing added protection from coil impact damage due to possible contact of the coil by the metallic structure under test; and (6) with regards to multi-frequency operation from a single transducer, with coil impedance properties being stabilized, it becomes practical to drive a single transducer at multiple frequency bands, resulting in a single transducer transmit and receive pair capable of inspecting multiple sample cross-sections or depths.